An oscillator unit for use in detecting an angular rate of rotation is disclosed in Japanese Laid-Open Patent Applications No. 52,410/1987 and No. 240,649/1993 and published UK Patent Application GB 2 266 149 A.
In a device for detecting an angular rate of rotation as is disclosed in the Applications cited above, an oscillator such as a piezoelectric element is driven for oscillation at a frequency (resonant frequency) which coincides with the natural frequency thereof, and a phase difference between a voltage applied to an exciting electrode of the oscillator and a voltage which is produced at a detecting electrode is determined. When the oscillator rotates or angularly moves, the phase difference assumes a value which corresponds to the angular rate.
It will be seen that the natural frequency of an oscillator slightly varies from element to element due to a variation in the size during the manufacturing process, and also fluctuates under the influence of an ambient temperature or the like. An oscillator which is used in a device for detecting an angular rate of rotation generally comprises an exciting electrode to which a driving or exciting voltage is applied, a feedback electrode at which a voltage phase displaced about 90.degree. with respect to the voltage applied to the exciting electrode is developed when the oscillator is oscillating at its natural frequency, and a detecting electrode at which a voltage is developed having a phase difference with respect to the exciting voltage which corresponds to the angular rate of the oscillator. An electrical circuit which drives the oscillator for oscillation includes a positive feedback loop in which a voltage developed at the feedback electrode is phase shifted and amplified before it is applied to the exciting electrode. A phase difference between the voltages at the exciting and the feedback electrode significantly deviates from 90.degree. in accordance with a displacement of an actual oscillation frequency with respect to the resonant or natural frequency. In addition, the amplitude of the ocsillation fluctuates. A displacement of the oscillation frequency causes an increased error in determining the angular rate.
Accordingly, in Japanese Laid-Open Patent Application No. 52,410/1987 and in the cited UK Application GB 2,266,149 A, PLL (phase locked loop) circuit is employed to control the oscillation of the oscillator so that the natural frequency is maintained. Specifically, the oscillation frequency of VCO (voltage controlled oscillator) is automatically adjusted so that the phase difference between the exciting voltage and the feedback voltage (namely, the voltage developed at the feedback electrode) is equal to 90.degree., or so that the oscillation occurs at the resonant frequency.
In Japanese Laid-Open Patent Application No. 240,649/1993, a voltage developed at the feedback electrode of the oscillator is phase shifted in a phase shifter and then amplified before it is applied to the exciting voltage (in a positive feedback manner) for driving the oscillator.
To maintain a stable oscillation of the oscillator at the resonant frequency, it is desirable that PLL circuit be employed to control the drive of the oscillator, as described in Japanese Laid-Open Patent Application No. 52,410/1987 and the cited UK Application GB 2 266 149 A. However, such a device may fail to start the oscillation immediately after the power is turned on. For example, if VCO should begin to oscillate at a frequency which is displaced from the resonant frequency of the oscillator immediately upon turn-on of the power, the amplitude of oscillation of the oscillator is reduced while the amplitude of undesired oscillation components (noises) increase relative to the resonant frequency component, so that a noise signal having frequencies other than the resonant frequency may be applied to a phase comparator, which forms part of the PLL circuit, or no signal may be applied thereto. As a consequence, the oscillator may oscillate out of the resonant frequency or may fail to oscillate entirely.
Referring to FIGS. 3a and 3b which show an oscillator 2, a phase difference of about 90.degree., at the resonant frequency of an oscillator 2, is developed between a voltage applied to exciting electrodes 5a, 5b of the oscillator 2 and a voltage developed at feedback electrodes 4a, 4b. Accordingly, by phase shifting the voltage developed at the feedback electrodes 4a, 4b by 270.degree. to apply a positive feedback to the exciting electrodes 5a, 5b, the oscillator 2 may be driven into oscillation. However, immediately after the power is turned on to the circuit, for example, signals having frequencies which are displaced from the resonant frequency may appear at the feedback electrodes 4a, 4b of the oscillator 2. In such instance, the phase difference between the voltage applied to the exciting electrodes 5a, 5b of the oscillator 2 and the voltage developed at the feedback electrodes 4a, 4b will be offset from 90.degree., resulting in a failure to apply a positive feedback to the exciting electrodes 5a, 5b and hence to maintain the oscillation. In this manner, the circuit may fail to start the oscillation. In addition, if the resonant frequency of the oscillator is offset from a reference value or designed value due to variation in the response of the oscillators 2 or due to a temperature change, the magnitude of a phase shift caused by electrical circuit elements contained in a closed loop (positive feedback) circuit may mismatch the oscillation frequency of the oscillator, i.e., deviating from a phase difference of 270.degree., causing a failure to oscillate.